Tunable filter

A tunable filter, having an input end and an output end, comprising:

• a first circuit and a second circuit, wherein both of the first circuit and the second circuit include:
• a first tunable capacitor, having a first end and a second end;
• a first inductor and a second inductor, wherein the first inductor and the second inductor are connected in serial and further connected with the first tunable capacitor in parallel, and a mutual induction is generated between the first inductor and the second inductor;
• and
• a second tunable capacitor, having a first end and a second end, wherein the first end of the second tunable capacitor is connected between the first inductor and the second inductor, and the second end of the second tunable capacitor is grounded;
• wherein the first end of the first tunable capacitor of the first circuit is coupled to the input end of the tunable filter through a first coupling tunable capacitor, the second end of the first tunable capacitor of the first circuit is coupled to the first end of the first tunable capacitor of the second circuit through a second coupling tunable capacitor, and the second end of the first tunable capacitor of the second circuit is coupled to the output end of the tunable filter through a third coupling tunable capacitor;
• wherein a ratio of a capacitance of the first tunable capacitor to a capacitance of the second tunable capacitor is 0.08˜0.12 in the first circuit, and a ratio of a capacitance of the first tunable capacitor to a capacitance of the second tunable capacitor is 0.08˜0.12 in the second circuit.